Corn rootworm insecticide results

Labeled rates of corn rootworm insecticides were evaluated across Iowa during the past three years in six locations per year. Five of the fields were conventional-till (chisel plowed in the fall and field cultivated in the spring). The sixth field was no-till. Tests were conducted in various soil types and under various moisture conditions. All products were evaluated in replicated side-by-side tests for their ability to protect corn roots from corn rootworm larval injury.

[1]

Heavy feeding injury caused by rootworm larvae.

[2]

Corn rootworms can injure corn roots.

Corn roots were rated for rootworm injury by using the Iowa 1-6 scale:

1 = no visible damage or only a few minor feeding scars

2 = some roots with feeding scars but none eaten off to within 1.5 inches of the plant

3 = several roots eaten off to within 1.5 inches of the plant but never the equivalent of an entire node of roots destroyed

4 = one node of roots destroyed or the equivalent

5 = two nodes of roots destroyed or the equivalent

6 = three or more nodes of roots destroyed

Insecticide consistency ratings were applied to the root ratings. To prevent yield loss, an insecticide should protect the roots so that no more than approximately one-third of a node of roots is eaten. If an insecticide gave this level of protection every time, it would be 100 percent consistent, indicating that the root rating was always at 2.99 or below. Basically, the higher the consistency, the better the performance.

Roots from untreated check rows, in both tillages, suffered moderate to heavy injury and averaged about 1.5 nodes (circles) of roots destroyed. Any tests that did not challenge an insecticide's performance (no obvious root pruning in the untreated rows) were excluded from the analysis.

All granular products were evaluated in-furrow or a T-band except Thimet. Thimet is the only granular insecticide in these tests that cannot be placed in-furrow because of possible seed or seedling phytotoxicity. Performance results are shown in the tables.

Table 1. Granular corn rootworm insecticide performance (1997-1999).

Insecticide

Placement

% Consistency

Root Rating

(1-6 scale)

Aztec 2.1G

Furrow

91a

2.1a

Force 3G

Furrow

88a

2.2a

Force 3G

T-band

86a

2.2a

Counter 20CR

Furrow

84a

2.3a

Counter 20CR

T-band

80ab

2.2a

Aztec 2.1 G

T-band

78ab

2.3a

Fortress 5G

Furrow

65bc

2.6b

Lorsban 15G

T-band

63bc

2.7b

Lorsban 15G

Furrow

57c

2.8b

Fortress 5G

T-band

57c

2.9b

Thimet 20G

T-band

56c

2.9b

Untreated Check

--

0d

4.4c

Consistency equals the percentage of times an insecticide treatment prevented potential yield loss (insecticide root rating less than or equal to 2.99 when untreated check root rating greater than or equal to 3.00).

Data from 17 tests (81 replications) at various locations throughout Iowa. Means within a column followed by the same letter are not significantly different (P < 0.05, Ryan's Q test).